Volume 24, Number 4—April 2018
Genetic Characterization of Enterovirus A71 Circulating in Africa
Enterovirus A71 (EV-A71; species Enterovirus A, genus Enterovirus, family Picornaviridae) is a common etiologic agent of hand, foot and mouth disease in young children. In addition, EV-A71 has been associated with severe and sometimes fatal neurologic diseases, including aseptic meningitis, encephalitis, and poliomyelitis-like acute flaccid paralysis (AFP) (1,2).
EV-A71 is classified info 7 genogroups (A–G). Genogroup A includes the prototype strain BrCr that was isolated in the United States in 1969 (1,2). Most EV-A71 isolates belong to genogroups B or C, which are each further divided into subgenogroups (1,2). Subgenogroups B4, B5, and C4 are mainly restricted to countries in Asia, whereas C1 and C2 circulate primarily in Europe and the Asia-Pacific region (1). Genogroup D and the newly proposed genogroup G appear to be indigenous to India, whereas genogroups E and F were recently discovered in Africa and Madagascar, respectively (3).
Although EV-A71 has been reported in many parts of the world, its epidemiology remains largely unexplored in Africa. An EV-A71 outbreak was documented in 2000 in Kenya, where HIV-infected orphans were infected by EV-A71 genogroup C (4). Several AFP cases have been associated with EV-A71 infection during 2000–2013 throughout Africa: in Democratic Republic of the Congo (5) (2000, n = 1); Nigeria (6) (2004, n = 1, genogroup E); Central African Republic (7) (2003, n = 1, genogroup E); Cameroon (8) (2008, n = 2, genogroup E); Niger (9) (2013, n = 1, genogroup E); and Senegal, Mauritania, and Guinea (9) (2013–2014, n = 3, subgenogroup C2). Four additional EV-A71 strains were obtained from captive gorillas in Cameroon during 2006–2008 (n = 2, genogroup E) (10) and from healthy children in Nigeria in 2014 (n = 2, genogroup E) (11). Molecular identification of all these isolates was based only on the analysis of sequences of the viral protein (VP) 1 capsid protein region.
Recombination events may be associated with the emergence and global expansion of new groups of EV-A71 that have induced large outbreaks of hand, foot and mouth disease with high rates of illness and death (12). For EV-A71, genetic exchanges have been described both within a given genogroup and with other types of enterovirus A (EV-A), usually in nonstructural genome regions P2 and P3 (1,12,13). However, before 2017, no complete genome sequence of EV-A71 detected in Africa has been reported, diminishing the power of such analysis. We examined the complete genome of most EV-A71 isolates reported to date in Africa to characterize the evolutionary mechanisms of genetic variability.
We sequenced the full genome of 8 EV-A71 isolates obtained from patients with AFP (Table): isolates 14-157, 14-250, 13-365, 13-194, and 15-355 from West Africa and isolates 08-041, 08-146, and 03-008 from Central Africa. We isolated and typed these isolates as previously described (7–10) and obtained nearly complete genomic sequences using degenerated primers (13) and additional primers designed for gene-walking (available on request) or unbiased sequencing methods (14). We determined the 5′-terminal sequences by means of a RACE kit (Roche, Munich, Germany). We deposited viral genomes in GenBank (accession numbers in Table) and submitted sequence alignments under BioProject PRJNA422891. We aligned sequences using ClustalW software (http://www.clustal.org).
To investigate the genetic relationship between Africa and global EV-A71 isolates, we constructed subgenomic phylogenetic trees based on the P1, P2, and P3 regions of the genome (Figure 1). We identified viral isolates showing related sequences in 1 of these 3 regions by BLAST search (http://www.ncbi.nlm.nih.gov/BLAST) and included them in the corresponding datasets used for analyses. We completed these datasets with a representative global set of EV-A71 sequences available in GenBank and belonging to the different EV-A71 genogroups (Technical Appendix[PDF - 115 KB - 1 page]). As expected, in the structural P1 region, the 8 isolates we studied clustered within their respective genogroups (C1, C2, and E), previously established by VP1-based typing (Figure 1, panel A). In particular, the isolates of genogroup E consistently clustered together (bootstrap value 100%), confirming their belonging to the EV-A71 type and their divergence from the other isolates belonging to the common genogroups A, B, and C. Analysis of the nonstructural P2 and P3 genome regions were in agreement with these data. However, the genetic heterogeneity, <12%, observed among the complete genome of genogroup E sequences highly suggested that they have circulated and diverged for years in a large geographic area in Africa. The unique Africa EV-A71-C1 strain clustered with other C1 strains originating worldwide, regardless of which genome region we analyzed. In contrast, the nonstructural sequences of Africa EV-A71 isolates of subgenogroup C2 did not cluster with their non-Africa C2 counterparts or with any of the existing EV-A71 genogroups. The incongruent phylogenetic relationships of Africa C2 strains in the different regions of the genome suggested that recombination events have occurred during evolution.
To examine further recombination events, we analyzed EV-A71-C2 study strains by similarity plot against potential parental genomes (Figure 2). This analysis showed that sequences 14-157, 14-250, and 15-355 had high similarity (>95%). By contrast, 13-365 diverged from the other C2 isolates around nt 5600 in the P3 region, suggesting a recombination breakpoint. The analysis showed high sequence similarity (>97%) between the studied EV-A71-C2 isolates and other subgenogroup C2 strains over the P1 capsid region. Conversely, in the noncapsid region, sequence similarity between Africa EV-A71-C2 isolates and classical subgenogroup C2 isolates (e.g., GenBank accession no. HQ647175) was much lower (66%–77%). This finding confirmed a recombination event of the Africa EV-A71 C2 lineage with an unknown enterovirus, the most likely breakpoint being located between nt 3596 and 3740, within the 2A gene. Sequence identity of EV-A71-C2 study strains with their closest related viruses (coxsackievirus A10 [CV-A10], CV-A5, EV-A120, and EV-A71 genogroup E strains) in the 3′ half of the genome was <87.7%. Of note, we found much higher sequence identity with the full-genome sequence of CV-A14 isolate in our database, obtained in 2014 from a patient with AFP in Senegal (15). This strain features a high similarity value (>97%) with the 3′ half of the genomes of EV-A71-C2 West Africa strains (Figure 2), indicating that their P3 regions share a recent common ancestor. Because these strains belong to 2 different types, this finding strongly suggests that genetic exchanges occurred through intertypic recombination. This result cannot be a result of cross-contamination during the sequencing process because the CV-A14 and EV-A71 isolates were sequenced on 2 different platforms.
Genogroup E was previously identified and characterized only on the basis of VP1 analysis (3). This study confirms the circulation in West and Central Africa of EV-A71 isolates belonging to the new genogroup E on the basis of the characterization of whole genomes. The divergence among isolates indicates that this genogroup has been extensively circulating in Africa. We also suggest that the common ancestor of EV-A71-C2 strains in West Africa has undergone recombination with >1 EV-A circulating in Africa. Genogroup E and recombinant C2 appear to be indigenous to Africa; they have not yet been detected elsewhere. Further exploration of environmental or clinical samples using deep sequencing technology would be of interest to determine the extent of EV-A71 circulation in Africa in the absence of AFP cases. Systematic surveillance based on full-genome sequencing could also serve to monitor these viruses for potential recombinations and to study their role in the emergence of new EV-A71 variants in Africa.
Dr. Fernandez-Garcia is a scientist with a PhD degree in virology and is involved in research and surveillance of enterovirus infections at Institut Pasteur of Dakar, Senegal. Her research interests include infectious disease epidemiology and public health microbiology.
We thank Karla Prieto and Catherine Pratt, who assisted in obtaining nearly complete genomes of West Africa strains, and Joseph Chitty for analysis of the next-generation sequencing data.
The next-generation sequencing equipment at Institut Pasteur of Dakar was provided by the Defense Biological Product Assurance Office under the Targeted Acquisition of Reference Materials Augmenting Capabilities Initiative. This work was supported by the IPD, the Pasteur Institute’s Transverse Research Program PTR484, Actions Concertées Inter-Pasteuriennes A22-16, Fondation Total Grant S-CM15010-05B, Roux Howard Cantarini postdoctoral fellowship, and Grant Calmette and Yersin from the International Directorate of the Institut Pasteur.
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Technical AppendixCite This Article
1These first authors contributed equally to this article.
2These senior authors contributed equally to this article.